Abstract
Human SAMHD1 (HsSAMHD1) is a metal-dependent hydrolase that plays a key role in HIV restriction, innate immunity, cancer, and the cell cycle by depleting cellular dNTP levels. Recently, SAMHD1 was proposed to be the molecule responsible for neurological complications associated with COVID-19. Despite being a sophisticated phosphohydrolase, the underlined molecular mechanisms are still not entirely understood. SAMHD1 orthologs have been recently discovered in diverse organisms such as plants and human parasites as unrecognized players in key biological processes. These orthologs have yet to be biochemically characterized and thus their specific roles remain unknown creating an unmapped protein space within the SAMHD1 superfamily. In this study we attempt to determine the molecular mechanisms of a SAMHD1 ortholog from Rhizophagus irregularis (RiSAMHD1) to i) gain insight into its role in modulating dNTP pools in cellulo, and ii) serve as a simpler functional template for understanding HsSAMHD1 regulation and activity. We show that RiSAMHD1 assembles a diiron metallocofactor and can hydrolyze all four canonical dNTPs in the presence of excess divalent transition metals. We also show that RiSAMHD1 activity is not apparently dependent on allosteric G-containing effectors. These preliminary results dissect the molecular details of RiSAMHD1 dNTP hydrolysis mechanism and show important differences from that of HsSAMHD1, which offer a first glimpse into the functional proteome of the SAMHD1-like landscape.